In motor vehicles, for supplying the electrical on- board system consumers, rotary current generators are employed, which are driven by the internal combustion engine. These rotary current generators, such as claw pole generators, are connected to the direct voltage on-board vehicle electrical system via a diode rectifier bridge. The voltage level to which the generator is controlled is at present approximately 14 V. The power output by the rotary current generators is controlled via the magnitude of the exciter current that flows through the exciter winding. As the controlled variable, the on-board electrical system voltage or the output voltage of the generator is typically used.
Since the electrical power consumption required in the vehicle electrical system is considerable and will increase still further in the future, it is presently already conventional to construct a vehicle electrical system as a dual-voltage vehicle electrical system, in which one voltage is then about 14 V and the other is about 42 V. Furnishing the higher voltage is done with the aid of a direct voltage converter, ground-connected to the generator, and this voltage converter acts as a step-up converter. A vehicle electrical system with at least one generator and one downstream direct voltage converter is known for instance from German Published, Unexamined Patent Application DE-OS 196 459 44. Both the controlling of the generator and the triggering of the direct voltage converter are done in this known on-board electrical system, with the aid of its own control unit, which processes the supplied information and outputs trigger pulses accordingly.
Another vehicle electrical system with a generator and a downstream step-up converter is known from German Patent DE-P 198 455 69. In this on-board electrical system, a claw pole generator is employed for supplying the on-board electrical system with 42 V. The generator terminals are connected, via a rotary current bridge rectifier, to an intermediate circuit, Connected downstream of it is a step-up converter for increasing the power of the generator. The power portion substantially comprises the generator. The output side of the generator supplies an intermediate circuit via a rotary current bridge rectifier. This intermediate circuit is connected to the on-board electrical system via the step-up converter. The step-up converter has no capacitive reactance of its own but instead uses the phase inductance of the generator as reactance. Thus this step-up converter makes do with a minimum number of components.
It is an object of the present invention to provide an improved method for optimal control of a generator with a voltage converter, especially for supplying electrical power in an on-board vehicle electrical system, of the above-described type, which makes the highest possible power output of the generator possible, even at low rpm.
It is another object of the present invention to provide an improved device for optimum control of a generator with a voltage converter, especially in an on-board vehicle electrical system, of the above-described type, which provides the highest possible power output, even at low rpm.
The method according to the invention for controlling a generator including an exciter coil and provided with an associated voltage converter operating as a step-up converter, includes the steps of:
a) performing a first control process for controlling the generator when an output voltage of the generator is below a normal operating voltage range, the first control process comprising increasing an exciter current flowing through the exciter coil until the output voltage is in the normal operating voltage range or increasing the exciter current until the exciter current reaches a maximum current value and then increasing the output voltage with the help of the step-up converter until the output voltage is in the normal operating voltage range, so that an output power of the generator is maximum;
b) performing a second control process for controlling the generator when the output voltage is within the normal operating voltage range, the second control process comprising clocking the exciter current to regulate the output power of the generator; and
c) performing a third control process for controlling the generator when the output voltage exceeds the normal operating voltage range, the third control process comprising limiting the output voltage by means of a P controller via the step-up converter in response to an overvoltage condition. The device according to the invention for controlling a generator including an exciter coil and supplying a load includes
a voltage converter for converting generator output voltage connected with the generator, the voltage converter operating as a step-up converter;
a controller for controlling an exciter current flowing through the exciter coil of the generator;
a subsidiary controller for controlling the step-up converter;
wherein the subsidiary controller and the controller for controlling an exciter current are connected with each other for exchange of information and either the subsidiary controller or the controller for controlling the exciter current acts to control the output voltage of the generator according to a value of the output voltage.
The device and method according to the invention for controlling a generator with an associated voltage converter have the advantage that a power output of the generator is possible even at rpm levels at which the output voltage of the generator would not yet suffice to supply the on-board electrical system directly. It is also advantageous that at rpm levels that cause higher output voltages of the generator, a control can be performed that makes a maximum power output possible.
Advantageously, by means of the voltage converter acting as a step-up converter, the generator voltage is freely adjusted, depending on the required value, between zero and the desired maximum value, which for instance is 42 V. By this voltage adjustment it is possible to operate the generator at a point of maximum power output, even in the lower rpm range. This maximum output, also called the tangent output, is only a function of the rpm for a given generator and maximum excitation. At higher rpm levels, the step-up converter can no longer be used to increase the power; then the generator is advantageously controlled in its output power via the exciter current. At higher rpm levels, the same control strategy for the generator as is currently conventional for present generators is thus advantageously employed.
To protect against overvoitages, the step-up converter itself is advantageously used, By closure of the switch or switches of the step-up converter at overvoltages, the power output of the generator to the electrical system can advantageously be prevented. Thus the overvoltages and in particular the duration of the overvoltages after a load dump can be reduced. The Zener diodes that are currently usual in the rectifier bridge can then be replaced by conventional diodes. This is especially advantageous since at the higher voltage levels of up to 42 V, for which the generator is designed, there are presently no suitable Zener diodes available.
In preferred embodiments of the device and method according to the invention the step-up controller comprises means for controlling the terminal voltage of the generator by changing a duty factor of the step-up converter, which determines a ratio of voltage at an output of the step-up converter to voltage at an input of the step-up converter.
In preferred embodiments of the device according to the invention the subsidiary controller includes a P controller that rapidly responds to overvoltage and signals the step-up controller to limit generator output voltage.